Genetic Basis for the Development of Scurs in Bos Indicus-Influenced Crossbred Cattle

Abstract

Scurs are corneous growths that develop at the same location on the skull as horns. Typically, scurs do not fuse with the frontal skull and they range from buttons to horn-like structures. Scurs are epistatic to horns and only cattle that are heterozygous at the polled locus can grow scurs. Scurs are also more frequent in males than females, suggesting sexual dimorphism. The genetic mechanisms for development of scurs and their morphology is still not clear and the mode of inheritance of scurs is debated. Therefore, the primary objective of this research was to study the mode of inheritance of scurs and better understand the genetic mechanisms behind scurs. We used heterozygous polled progeny from Bos taurus-Bos indicus F2 and reciprocal backcross mapping populations. Phenotypic records and SNP genotypes were collected for cattle that were at least 18 months of age. We identified 3 types of scurs based on anatomy that can be seen as milestones in the development of horns. Two previously proposed inheritance models do not fit our populations and we found that the presence of scurs (Sc) is dominant over the absence of scurs (sc), and we suggest the presence of sheath (sh) is recessively inherited. Genome-wide association studies identified a major locus on BTA 12 that is adjacent to RXFP2 to be associated with presence of scurs and presence of sheath. We also identified a locus on BTA 17 that is adjacent to RXFP1 associated with presence of scurs in males, suggesting there are genetic compensation effects from RXFP1 for development of scurs in male cattle. Spatial transcriptomics revealed scurs and horns share a similar expression profile, suggesting scurs are incomplete horns that due to mutations affecting genes in the developmental pathway stopped at different points in development.